It's actually pretty good for emulation, I've ported a whole lot of stuff to it... dosbox, temper (pc engine), mame4all, nethack, rogue and sms_sdl (sega master), castaway (atari st) to name a few.
The keyboard on it is nice for emulating computers, such as dosbox. Controls are okay for arcade stuff, not brilliant, but pretty good. The battery life, like all jz47xx device is amazingly good.
Any yeah, they are super rare. I bought mine originally from eBay many years ago - was lucky enough to find one after they were released.
I wish someone would release an updated version of this with a jz4770 inside! Would be very nice!